1. Field of the Invention
The present invention generally relates to broaching machinery, such as the type used to broach large diameter internal shapes, which includes an arbor with slots or the like formed in its outer surface or in a shell fitted over the arbor for receiving a number of broaching inserts. More specifically, this invention relates to such a broach apparatus wherein the slots are precision formed in a number of annular members whose axial lengths are sufficiently short to permit manufacture of the annular members by electrical discharge machining (EDM) technologies, so as to result in a more accurate and highly repeatable placement of the inserts relative to the arbor and, therefore, relative to a workpiece.
2. Description of the Prior Art
Various types of internal shape-type broaches have been developed over the years for the purpose of machining splines and various other forms on workpieces whose configurations permit a broach to be passed completely through the workpiece. Early forms of such broaches include solid broaches formed from one-piece bar stock, which typically have a number of integrally-formed teeth. More recently, shell internal broaches have been developed, which differ from solid broaches in that they include teeth formed on, or inserts mounted to, a removable shell secured on an arbor. Shell internal broaches are superior to solid broaches with respect to the ability to replace only the shell instead of the entire broach when the teeth or inserts are worn or damaged. However, solid broaches are generally able to provide greater dimensional accuracy and concentricity than shell internal broaches, due to normal dimensional variations being compounded by the additional number of pieces required for shell internal broaches.
A conventional shell internal broach is shown in FIGS. 1 and 2. Generally, one or more tubular-shaped shells 12 are mounted on an arbor 10 so as to abut an enlarged portion 22 at one end of the arbor. The shells 12 are secured to the arbor 10 with a nut 26 threaded onto a threaded portion 24 located at an end of the arbor 10 opposite the enlarged portion 22. Each shell 12 serves to secure a set of broaching inserts 16 on their periphery, such that each set of broaching inserts 16 is separately mounted to the arbor 10. Accordingly, the ability to remove one of the shells 12 and replace it with another having new or reground broaching inserts 16, which are of a different diameter and/or a different configuration, provides a versatility which is not possible with solid broaches.
The broaching inserts 16 are received in a corresponding number of ground slots 14 or other precision manufactured shapes formed in the peripheral surface of the shells 12. The broaching inserts 16 may be secured in the slots 14 with fasteners, such as the bolts 18 shown or locking wedges (not shown), as is known in the art. Preferably, a series of shaving rings 20 are also mounted on the arbor 10 behind the broaching inserts 16, such that the shaving rings 20 perform work on a workpiece after the broaching inserts 16 have completed their work, so as to remove errors of form introduced into the workpiece by the broaching inserts 16.
As noted above, shell internal broaches of the type shown in FIGS. 1 and 2 have a significant shortcoming in that dimensional accuracy and concentricity are less than that of solid broaches as a result of their additional number of components. This is true even though precision machining and grinding operations are employed to form the slots 14 on the shells 12. Because of this, shell internal broaches constructed in accordance with the prior art are relatively expensive to manufacture, due to the additional, expensive and time-consuming manufacturing operations required to sufficiently achieve the precision desired of the shells 12 and slots 14. The costs associated with shell internal broaches are further increased by expenses incurred in their maintenance, such as the need to regrind the shells 12 and slots 14 as they become worn or damaged.
Further, the machining and/or grinding operations necessitated by the shell internal broaches taught in the prior art do not lend themselves to existing automated or numerically-controlled machining technologies, such as electrical discharge machining (EDM) technologies, and in particular, travelling-wire EDM technologies. Travelling-wire EDM is a highly desirable manufacturing method in that accuracies of about .+-.0.013 mm (.+-.0.0005 inches) are readily achievable, with accuracies of better than about .+-.0.005 mm (.+-.0.0002 inches) being possible. However, the shells 12 used by the prior art are prohibitively long to permit the use of travelling-wire EDM technologies. As a result, the shells 12 and slots 14 must typically be ground to size, which is generally less precise than travelling-wire EDM.
Accordingly, what is needed is an internal broach apparatus which is capable of dimensional accuracies and concentricities comparable to that of solid broaches, yet retains the versatility and interchangeability of shell internal broaches. It would also be desirable that such an internal broach provide reduced manufacturing and maintenance costs, as compared to prior art shell internal broaches, while permitting the use of modern machining technologies, such as numerically-controlled travelling-wire EDM operations.